In conjunction with the processing of certain agricultural product, a plant fibrous material is formed which, in addition to the plant fibres, also contains impurities. As non-restrictive examples of such plant fibrous materials, mention might be made of a pulp constituting the residual product from the recovery of sugar from sugar beets and different types of products which normally constitute residual products after the processing of tuber or root vegetables or of fruits. Examples of such tuber and root vegetables are potatoes and carrots, and examples of such fruits are apples, citrus fruits etc. In recent years, plant fibres, for instance from plant fibrous materials of the above kind, have been put to increasing use for human consumption, primarily because it has been found that, by the consumption of such fibres, the body is supplied with important components, that the fibres have a low calorie content and that they stimulate the peristalsis in the intestine, viz this latter having obvious medical advantages. Materials containing plant fibres intended for human consumption are often named dietary fibre products. The most common type of dietary fibre consists of cellulose, hemicellulose, pectin and lignin. That part of the plant cell walls which is not digested or broken down by the enzymes of the intestinal tracts constitutes dietary fibre products.
As intimated above, the water insoluble residual products which are formed on the comminution extraction of vegetables, for example pulp from sugar beets, potatoes, citrus fruits, apples etc., are frequently used as raw material in the production of dietary fibre products. In such instance, the water-insoluble residual products generally include parts of the starting material, foreign materials and/or substances formed during the processing thereof which are undesirable in the finished product. Non-limiting examples are substances imparting an unpalatable flavor or odor (such as aldehydes, ketones, geosmin, etc), peel, cores, burnt particles, soot flakes and the like, as well as earth particles such as sand. The generic term impurities will be employed herein to define such undesirable parts, materials and substances.
In order to be able to employ the plant fibres in the diet, for example as a dietary supplement, it is necessary that the impurities have been removed from the plant fibres beforehand.
The presence of sand particles constitutes a particular problem in the recovery of plant fibres of the kind mentioned above in that said particles adhere to the plant fibres or are embedded therein. Such is for example the case in the sugar beet pulp which constitutes the residual product in the extraction of sugar from sugar beets.